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Steel design Topics related to the design of structural steelwork to BS449 or BS5950

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Old 17th August 2011, 09:20 AM   #1
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Default help requested !!!!


I have designed a beam which takes the back of an extension. It spans 5.2m and takes a point load at 2.1m from R1 and I have spliced it 1.8m from R2. I did not use the point load from restaint and have taken 1.2L + 2D as effective length and the beam is bolted to stanchions each end which are still plumb.

Had a phone call last week to say the beam has deflected 18mm at the splice

The calculated deflection was 14mm so initially thought that the splice had slipped.

Went up to site yesterday and the beam is buckling and the deflection at the splice has risen to about 23mm. This is far in excess of the calculated deflecton and all the load has not been applied yet

I came back and re did the calc from scratch including working the point load out again etc and the beam is the same size (305 x 305 x 97 UC) and the splice also works

As the deflection is at the splice I worked back and hand checked unfactored stresses in flange and plates etc and they are OK (the flange stress is 113 n/mm2 unfactored by the way)

In desperation I then re did the calculation and splice with eurobeam and it all works OK

So in theory the beam should be OK and under its current load deflecting about 12mm but its twice this in practice

There is something fishy about the beam as it measures 305mm wide and 315mm deep with a flange thickness of 14mm which does not seem to match the size of a 305 x 305 x 97 UC but I am assured its within rolling tolerances

I cannot see what is going on and the beam is just not working as it should be and wondered if anyone had an idea I am pulling my hair out and am now thinking its the steel itself and maybe its second hand and been overstressed etc

Any comments greatfully received


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Old 17th August 2011, 01:41 PM   #2
Tony Bryer
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Location: Melbourne, Australia
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Default Re: help requested !!!!

The key question, based on what you've said, is what section size has actually been used? Unfortunately less knowledgeable builders tend not to realise that section families come in different rolling weights and don't appreciate the significance of a the specified rolling weight. However this shouldn't be a problem for a 305x305x97 as this is the lightest 305x305: depth 307.9, width 305.3, web 9.9, flange 15.4. Tolerance for depth +4 -2, width +/- 4, flange +2.5 -1,5, web +/- 1.0, source

Also, to ask an obvious question, as you say it's a secondhand member, is it a UC (inner and outer faces of flange parallel) or an older RSC section (inner face tapered)?
Tony Bryer, Greentram Software Pty Ltd
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Old 17th August 2011, 03:27 PM   #3
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Default Re: help requested !!!!

Hi Tony

I dont know if the steel is second hand I am just throwing in things I can think of

Thanks for details on tolerances I have worked out from the dims given by site that the depth is greater than the allowable so I will get the site to check the dims again


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Old 10th December 2011, 09:54 AM   #4
Colin Mitchell
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Default Re: help requested !!!!


May I also suggest you check the actual loading based on a survey of the materials and dimensions as used on site etc etc


PS: Maybe also check the bolts (size/grade) and hole sizes

Last edited by Colin Mitchell; 11th December 2011 at 08:58 AM. Reason: Update
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